The invention relates to a method and an arrangement for detecting overlapping flat mailpieces in a transport path for mailpieces that are transported vertically in succession.
A problem that arises in connection with separating devices consists in what are referred to as double-feed errors which occur as a result of the fact that in addition to the mailpiece that is to be removed from a stack a further mailpiece is picked off simultaneously. In sorting systems for flat mailpieces the undetected double feeds are particularly significant because they increase the missorting rate due to read errors by the automatic address identification system, but can also lead to a mailpiece jam with destruction of mailpieces as a consequence.
In a known solution movable mailpiece sections are temporarily deflected perpendicularly with respect to the conveyance direction. The presence of overlapping mailpieces is then detected by analyzing the flip-back behavior of the mailpieces (EP 0 650 911 B1). As a different offset of the overlapping mailpieces can be present, the deflection must be effected both on the front side of the mailpiece and on the rear side of the mailpiece, which results in increased complexity and expense. Also disadvantageous is the mechanical stress to which the mailpieces are subjected during the deflection. A further deficiency of this solution consists in the fact that the offset of the trailing edges must not fall below a minimum value (approx. 10 mm).
In another known solution the profile of the mailpiece height is measured and an overlap is reported in the event of a change in height (DE 196 25 044 A1).
There are possibilities of overlaps of two mailpieces occurring which cannot be detected by the above-cited solutions. This is the case in particular with identical, congruent mailpieces. However, it is also possible that a large mailpiece covers a small mailpiece in such a way that a double feed is transported onward undetected.
DE 12 10 601 A discloses a method and an arrangement for detecting mailpiece overlaps, wherein edges on both sides of the transport path are determined using edge detection measuring means. Congruent mailpieces lying on top of one another cannot be detected by this means.
A method and an arrangement for detecting mailpiece overlaps can be derived from U.S. Pat. No. 5,505,440 A, wherein the total length of a mailpiece or overlapping mailpiece is measured, subsequently overlapping mailpieces are displaced with respect to one another and then the total length is measured a second time. With this solution it is not possible to detect mailpiece overlaps if a large and a small mailpiece are overlapped and the small mailpiece is still in the shadow of the large mailpiece before and after the displacement.
A solution was also known (DE 101 42 331 C1) wherein after edges have been detected on both sides of the transport path and after the distances between the detected edges and the leading front edge measured by means of a light barrier in each case have been determined, both sides of the longitudinal sides of the mail are acted upon at different transport speeds, ensuring that the minimum gaps are maintained. The distances between the edges and the leading front edge are then measured a second time. A double feed is detected if there is a change in distance. In this case too, an overlap is determined by means of edge detection.
It is known from WO 01/89 724 A1 and JP 09-020 438 AA to detect overlapping mailpieces within a transport stage by means of a speed sensor arranged on one side of the transport path.
Transport paths are known from DE 102 12 024 A1 wherein the nominal speed of successive transport stages increases in the transporting direction.